In the electrostatic latent image process, an image is completed by forming an image on a photoreceptor by a charge property of individual toner particle, and then transferring the image onto a transfer material. It is thus known that the properties of the toner have a great effect on the image formation. However, the presence of defective toner particles that cannot be completely controlled in charge property cause disadvantages such as scattering, fogging and background stain. The occurrence of defective toner particles is attributed to insufficient charging and uneven charging caused by the variation of particle size or shape of toner particles.
As an image forming process involving the control over the particle size of the toner particles, there has been proposed an image forming process involving the use of a toner having: a weight-average particle size of from 4 to 11 μm; and a particle size distribution of from 3 to 15% by number of toner particles having a particle size of from 2.00 to 4.00 μm, from 8 to 19% by number of toner particles having a particle size of from 4.00 to 5.04 μm and 10% or less by volume of toner particles having a particle size of 12.7 μm or more (see, e.g., Reference 1). However, this proposal for image forming process merely defines the particle size of the toner particles to be used and has neither disclosure nor suggestion of charge property of each particle.
Since the sum of charged amount on the basis of the weight of the aggregation of toner particles has merely considered as the charged amount thereof, it has not been made possible to control the charged amount of individual toner particles. Therefore, the toner particles are attached to the edge (end portion) of the latent image more than to the center of the latent image, causing an edge effect that the development density rises more at the edge of the latent image than at the center of the latent image.
Also, there is a tendency in toner design that the particle size of individual toner particles are uniform as much as possible to have a sharp particle size distribution and control is made such that when the toner particle are charged, the charged amount of individual toner particles is uniform. For example, Reference 2 proposes a toner arranged such that the ratio of the volume-average particle size [μm] to the number-average particle size [μm] of toner particles is from 1.0 to 1.2 as determined by a coulter counter and the volume-average particle size of toner particles is from 3 to 25 μm. A toner having a relatively sharp particle size distribution shows a good uniformity in charge property and contains less particles having an opposite polarity, making it possible to eliminate the occurrence of fogging or scattering.
However, since the sum of charged amount on the basis of the weight of the aggregation of toner particles has merely considered as the charged amount thereof, it has not been made possible to control the charged amount of individual toner particles. As a result, during the reproduction of dot, toner particles which have been charged somewhat uniformly (i.e., toner particles having almost the same charged amount) can repel each other, causing “scattering” and hence causing uneven dot reproducibility.
Further, for controlling the charged amount of toner particles, there has been proposed a developing process which comprises friction-charging a developer at the contact area of a developer feeding member and a developer carrier which are moving on the surface thereof in the same direction, supporting the toner particles thus charged on the developer carrier, bringing the surface of the developer carrier on which the friction-charged developer is supported into sliding contact with a developer layer-forming member by which a developer has been retained by a minute electric field on the surface thereof to form a development layer free of unevenness, and then conveying the developer layer on the developer carrier to the position opposing an electrostatic latent image carrier (see Reference 3). It is proposed that this developing process can eliminate the amount of uncharged toner particles to provide a sharp distribution of charged amount (see, e.g., FIG. 5 of Reference 3).
[Reference 1]
JP-A-5-297631
[Reference 2]
JP-A-63-276064
[Reference 3]
JP-A-5-188757
In the related art, since the sum of charged amount on the basis of the weight of the aggregation of toner particles has merely considered as the charged amount thereof, the charged amount of the individual toner particles cannot be controlled. Accordingly, it has been made difficult to make an effective countermeasure against disadvantages of uneven image quality such as white blanks and edge effect. The term “white blanks” as used herein is meant to indicate a phenomenon that the central part of an image such as line image becomes white. This phenomenon occurs when a high pressure is applied to the central part to which a greater amount of toner particles are attached during transfer from the photoreceptor, causing the aggregation of the toner particles at the central part and hence making it impossible to transfer the toner particles. The term “edge effect” as used herein is meant to indicate a phenomenon that when a greater amount of toner particles having a relatively great particle size are attached to the edge (end portion) subject to stronger electric field than the center of the latent image in a patch pattern (in the case where a square is formed by a solid image or halftone image) or the like, the development density is higher at the edge than at the center of the latent image. Both the “white blanks” and “edge effect” occur when there occurs uneven development density in the latent image region.
In the related art, since the sum of charged amount on the basis of the weight of the aggregation of toner particles has merely considered as the charged amount thereof as described in Reference 3, the charged amount of the individual toner particles cannot be controlled. Accordingly, it has been made difficult to make an effective countermeasure against disadvantages of defective image quality called starvation. “Starvation” is a phenomenon that no toner particles are supplied into the area adjacent to e.g., solid image, line image or letters during the printing of a halftone image, causing the reduction of density at this area. This phenomenon occurs remarkably in a gradation priority mode in particular. This phenomenon occurs when toner particles are swept into areas having a high printing duty such as line during development step, making it impossible to develop the adjacent sites.
It is theoretically possible that development and image formation should be fairly conducted by the use of a toner having a constant ratio of charged amount to weight of the toner, that is, ideal charge property as in the related art. In actuality, however, the occurrence of defective toner particles that cannot be completely controlled in their charged amount unavoidably causes disadvantages such as starvation.
The invention has been worked out in view of these disadvantages. An object of the invention is to provide a developing process capable of providing a uniform development density free of unevenness over the entire latent image and an image forming process.
Also, other object of the invention is to provide a developing process and an image forming process capable of controlling disadvantages such as eliminating defective dot formation, scattering, fogging and background stain.
Further, other object of the invention is to provide a developing process and an image forming process capable of providing a high quality image without causing starvation in the formation of a halftone image or the like.